Geoscience Reference
In-Depth Information
t
c
= 21 min.
1.5
t
c
= 31.5 min.
60
50
40
30
10-yr. allowable
release rate = 24 cfs
20
10
0
t
c
0
15
30
45
Time (minutes)
60
75
90
Conversion of in.
2
to ft
3
= 9000 ft
3
/in.
2
Rainfall Avg.
Period (
T
)
21 min
30 min
35 min
40 min
50 min
60 min
Intensity
I
(in./hr)
Q
Peak
(cfs)
Area
(in.
2
)
Storage Volume
(ft
3
)
4.4
3.5
3.2
3.0
2.6
2.3
65
52
47
44
38
34
7.07
7.33
7.31
7.53
7.45
7.48
63,630
65,970
65,790
67,770*
67,050
67,320
FIGURE 25.15
Modified ration method runoff hydrograph, 10-year post-developed condition.
25.7.7.1 Storage Volume
The runoff hydrograph developed with the modified rational method, critical storm duration will
be either triangular or trapezoidal in shape. The outflow hydrograph of the basin is approximated
by a straight line starting at 0 cfs at the time
t
= 0 and intercepting the receding leg of the runoff
hydrograph at the allowable discharge (
Q
o
).
Note:
The straight-line representation of the outflow hydrograph is a conservative approximation
of the shape of the outflow hydrograph for an orifice control release structure. This method
should be used with caution when designing a weir control release structure.
The area between the inflow hydrograph and the outflow hydrograph in Figure 25.16 represents
the required storage volume. This area, the trapezoidal hydrograph storage volume, can be approxi-
mated using the following equation:
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